What shape is your city? It determines the amount of rain, science says

A new study focusing on rainfall patterns has come to an interesting conclusion: cities that are triangular receive the least amount of rain compared to square or circular ones. The study’s authors believe “the finding is valuable for sustainable and resilient city planning especially for those undergoing expansion.”

According to the authors, “results highlight the need for considering city shape for coastal urban planning, as a potential adaptation strategy to manage rainfall under future climate.”

The global climate crisis has created severe droughts in some areas of the world, while resulting in floods and landslides in others. The authors point out that “Record-breaking rainfall events have been increasingly observed during the past decades, with even more frequent intense rainfall expected under a warming globe in the future.”

The research, published in Earth’s Future journal, suggests that “Cities are particularly vulnerable to heavy rainfall because the expansion of impervious materials increases runoff volumes and subsequently elevates flood risk within urban areas.”

Moreover, the authors write, “by modifying land surface energy and moisture balances, cities themselves have notable impacts on regional rainfall through land-atmosphere interactions.”

This is the first investigation of the impact of city shape on urban rainfall in inland and coastal environments. The authors say that under calm synoptic conditions, “the city shape impact is much more evident in coastal environments.”

The authors explain that “city shape ranked by the rainfall amount and intensity in order from the largest to the smallest is: circular city, square city, and triangular city.”

“The relationship between city shape and urban rainfall is essential for sustainable and resilient city planning under climate change, especially for those undergoing expansion,” the authors write.

Their study, the authors say, aims to study city shapes in coastal and inland environments.

Looking at circular, square, and triangular cities (like Dallas, New York, and Los Angeles, respectively), the scientists “compared the results of Weather Research and Forecasting models and eddy simulations of air currents to see how shape could influence the weather. They also compared this to outer-city areas both inland and along coastlines,” IFL Science notes.

The researchers found that the influence of shape is greater in cities in coastal settings as airflow from the city interacts with the sea breeze: “The circular city receives the largest amount of daily rainfall, 22.0% greater than the amount over the triangular city. The difference in morning peak rainfall rate can reach 78.6% attributed to the differences in city shape.”

Overall, the shape that attracted the most rain was circular, followed by square, with the least and calmest variety of rain falling on triangular cities.

“With the consideration that climate change will intensify rainfall hazards in the future, global cities have spent vast resources to study and implement a variety of infrastructures as adaptation strategies,” the authors note.

“Our results identify a hitherto poorly understood but important role of urban layout especially in the coastal regions. Circular city shape shows potential risks of extreme rainfall and resultant flood risk.”

The authors admit to having some important limitations to the study:

“First, the timing and intensity of urban rainfall vary with the background wind.

“Second, focusing on the effect of city shape, land topography is largely ignored in our simulations.

“Third, urban rainfall is sensitive to the temporal evolution of surface heating, which may be affected by both the urban landscape (eg, city size, urban density) and surrounding environment (eg, soil temperature, sea surface temperature in case of coastal domains).”